Delete device messages asynchronously and in staged batches (#16240)

changelog.d/16240.misc

synapse/handlers/device.py

synapse/handlers/presence.py

synapse/handlers/sync.py

synapse/storage/databases/main/deviceinbox.py

synapse/storage/databases/main/devices.py

synapse/storage/databases/main/receipts.py

synapse/storage/engines/_base.py

synapse/storage/engines/postgres.py

synapse/storage/engines/sqlite.py

synapse/storage/schema/main/delta/48/group_unique_indexes.py

synapse/util/task_scheduler.py

tests/handlers/test_device.py